Hello, i just wanted to hear the ideas/experiences on some points about what is your though on the use of dice to reach top level of DMing.
1. Do you always keep hidden the dice results? If not, in which circumstances you disclose them?
If the players know a roll is getting made (most of them), I roll it in front of them.
Their characters live in the world, and should be able to tell things like "even though the ogre was really sloppy on his attack (a 7), it's immense strength was more than enough to bash through the paladin's armor and wound him". Seeing the dice helps get that information to the players should known from what their characters experience.
If they don't know that, for instance, someone is following them, I'll roll the stealth check secretly just not to let them know about the check.
2. Do you cheat to keep "balance" or "consistency"? Why and in which specific circumstances? Pay attention, please: this question doesn't aim to discuss about adjusting results for railroading purposes.
The DM shoudn't cheat. Ever. That said, some games like D&D grant the DM authority to do things that would be cheating in other games, like a Powered by the Apocalypse game that have very defined rules for the GM.
In a game like D&D, I will occasionally fudge, though usually something besides the dice. For instance, if the last monster in a combat gets hit and gets left with 1-2 HPs and there's no tension or realistic attrition* they could cause, I'll say the blow killed them. Pacing is more important than extending an already done-deal combat that has nop tension left in it. (Realistic attrition - if they go next and have something meaningful they can do, I'll keep them around.) Same as when a PC got a crit against their nemesis that left them with 4 HPs - you know what it's a better story to tell how you finished off your nemesis with a crit then some other PC dribbles in that last damage. Neither of these would change outcomes.
And there are legitimate decisions a DM can make that aren't cheating but will affect the outcome. For example they can decide how fast reinforcements arrive. A good way to keep a combat with high tension without haivng it either drag or become too overwhelming. (Only moderately overwhelming, so that the chgaracter have something dramatic to hope to overcome.) But not to save characters from themselves - if the PCs let the guards ring a gong they know they need to leave post haste if they don't want to handle every guard in the area showing up.
3. Do you tend to "overthrow" dice so that you have some results in advance? It happens a DM needs to have a result without showing to the players that he rolls the dice... Should you not "overthow", how do you manage to have a "secret roll"?
I roll dice secretly whenever I want. Sometimes it has meaning, like the instance above were someone is following them. Sometimes it's just me rolling a die for no reason.
I've seen DMs pre-roll a bunch of 20s. It's fine, though personally I feel I might be swayed knowing what numbers are coming up. I've seen GMs with a silent die roller on their phone so they can roll secretly without being obvious. Whatever works for you.